1. Field of the Invention
This invention relates generally to optical fiber devices and particularly to fiber based Mach-Zehnder interferometers (MZI), which are made insensitive to temperature changes and devices employing the same.
2. Prior Art of the Invention
Optical filters are frequently used in modem optical communication systems such as Dense Wavelength Division Multiplex (DWDM) systems. In these systems, a number of data channels share a single optical fiber as their transmission media and use a unique wavelength of light as their channel signature.
An optical waveguide MZI is composed of two optical splitters/couplers. Two lengths of optical waveguides or arms connect them to each other. When the arms of the MZI have different lengths, we have a so-called asymmetric Mach-Zehnder interferometer (AMZI). The optical waveguides referred to here are optical fibers with circular cross sections and/or planar optical waveguides with non-circular cross sections. A MZI made of optical fibers is called an all-fiber MZI.
Asymmetric MZIs show a periodic response as a function of wavelength. The period is a function of the length difference between the arms of the interferometer. As the length difference increases, the channel spacing decreases and, therefore, the wavelength selectivity increases. Asymmetric MZIs, once connected to each other as inter-leavers, can multiplex or de-multiplex a large number of optical signals of different wavelengths such as the standard ITU (International Telecommunications Union) grid wavelengths. An optical inter-leaver can separate the odd and even channels from a WDM signal consisting of several wavelengths.
One problem associated with fiber based AMZIs is their sensitivity to temperature, and the greater the length difference, the more severe is the problem. The problem originates mainly from a temperature-induced change in the optical path length of the fiber. As the temperature changes, the refractive index and the geometrical length of the fiber change. Consequently, a difference in the optical path lengths of two arms is created.
Several temperature compensation methods have been proposed to solve the temperature sensitivity of these photonic devices. However, most of these methods work within a limited range of temperatures and cannot be applied easily to the asymmetric MZI cases where large differences in the optical paths exist. Active temperature compensation of photonic devices is typically carried out by maintaining the temperature of the fibers"" environment above a chosen temperature (e.g., above 60xc2x0 C.). This is achieved by including a heater controller inside the package of the device. However, the high power demands of the active temperature compensation and its low reliability have made the search for passive methods an on-going effort within the photonic industry.
Temperature insensitive fibers can be built by a method that, for example, has been disclosed in the U.S. Pat. No. 5,018,827. In the named patent, an insensitive optical fiber is produced when an optical fiber core made of a first material is enclosed within a cladding made of a second material having a different coefficient of thermal expansion.
In a recent U.S. Pat. No. 6,081,641, a passive temperature compensating method is presented for a fused-fiber DWDM system. In this invention, two dissimilar materials with different thermal expansion coefficients are used to construct a fixture containing the DWDM device. By using this structure, it is possible to artificially create a negative coefficient of thermal expansion. The DWDM device is typically assembled on a pre-stressed fixture. However, the device can also be built under tension and then assembled on the relaxed bi-substrate fixture. In the former design, the whole assembly can exert tension on, or release tension from, the fiber. Temperature compensation is then established by adjusting the applied tension on the fused-fiber DWDM. It is shown that, as tension is relieved, the thermal drift due to an increase in temperature is compensated. Conversely, by increasing tension, wavelength shifts due to a decrease in temperature are compensated. By using such a temperature-compensating device, a bulky package is inevitable. In addition, dimensional design and choice of material can be demanding requirements.
An object of this invention is to provide a novel temperature insensitive asymmetric fiber based MZI.
Optical filters with sharp wavelength characteristics are vital components of WDM technology. Interferometer devices, and in particular fiber based interferometer devices such as the MZI, show useful filtering characteristics, are easily expandable, and exhibit low insertion loss. A fiber based optical MZI consists of two optical couplers or splitters with predetermined coupling or splitting ratios connected together through two lengths of optical fiber. In order to decrease the channel spacing between two adjacent channels of an interleaver response, the length difference (xcex94l) between the two arms should increase. As a result, the optical path length difference also increases, generating a higher sensitivity within the MZI to fluctuations in its temperature.
The challenge lies in correctly achieving the desired channel spacing. This is accomplished by measuring the correct xcex941 between the two arms connecting the two couplers of the MZI. As a result of the different optical paths between the two arms of the two couplers, a sinusoidal wavelength response can be obtained with low polarization dependence and low insertion loss. Using a precision reflectrometer or an optical spectrum analyzer, the difference between the two arms of the MZI can be measured to within xc2x110 xcexcm.
The object of this invention is consistent of two parts. In one part, a temperature insensitive MZI may be made using a specialty fiber as disclosed in U.S. Pat. No. 5,018,827. An insensitive optical fiber can be tailored such that temperature-induced changes in its geometrical length and in its refractive index offset each other in such a fashion that the optical path length is, for all intents and purposes, independent of temperature. By carefully choosing two different glasses for the core and cladding, and by appropriately adjusting their radii, the observed center wavelength shift sensitivity of the MZI filter due to temperature variations can be eliminated. It is noted this specialty fiber that may contain different dopant concentrations from that in the regular single mode fiber can be fusion spliced to a regular fiber with minimum loss. In the second part of this invention, a layer of a properly selected material is deposited onto a small section of one arm or both arms of the MZI to compensate for the temperature-induced variations.